1. Field of the Invention
The present invention relates to ion implantation, and particularly to a method of removing the electric charge accumulated on a semiconductor substrate in ion implantation.
2. Description of the Prior Art
The integration degree of integrated circuits has been increased about four times over the past three years. Improvements in integration degree have been achieved by making finely structured semiconductor devices. In order to make such a device extremely fine, it is necessary and effective to reduce the thicknesses of the insulating films used in the devices. For example, in a usual MOS type device, a silicon oxide film (SiO.sub.2) with a thickness of 100 .ANG. is generally used as the insulating film. The insulating film is formed by so-called ion implantation, in which positive ions are accelerated toward the substrate of the device and implanted therein at a high speed.
Positive electric charge therefore accumulates on the substrate (charge-up phenomenon), so that the insulating film is likely to be destroyed when an excessive electric stress is applied thereto. As the result, the reliability of the semiconductor device is degraded, and productivity thereof lowered.
To prevent the charge-up phenomenon, there is a known method in which an electron beam is irradiated to the substrate or ion beam by using an electron flat gun (EFG).
FIG. 1 shows a cross section of a conventional ion implantation apparatus. An ion beam 51 which emerges from an ion source (not shown) is accelerated, then implanted into a semiconductor substrate 53 mounted on a disc 52. In the same figure, electron flat gun (EFG) 54 prevents positive charge-up to be generated on the surface of the substrate 53 by the ion beam 51. In the electron flat gun 54, primary electrons generated from a tungsten (W) wire 55 are accelerated, then collide with a target 57, so that secondary electrons 58 are generated from the target 57. Thereafter, the secondary electrons 58 are introduced on the substrate 53, so that the positive electric charge having accumulated on the substrate 53 is neutralized. The amount of the secondary electron beam irradiated on the substrate 53 can be detected by monitoring the flow of secondary electrons 56 to the disc 52, further controlled by changing values of voltage 59 applied for generating the secondary electrons 56.
However, since the degree of charge-up varies with conditions of the surface structure of the substrate 53, it is very difficult to adjust most suitably the irradiation amount of the electron beam 56 given from the EFG 54 for satisfying all the surface conditions of the substrate 53. For example, when the amount of the irradiation of the electron beam 56 exceeds an amount just corresponding to the amount of charge-up, the surface of the substrate 53 is charged negatively. Moreover, in case of a CMOS type semiconductor device, the amount of the P channel portion thereof is different from that of the N channel portion. Therefore, it is impossible to prevent the charge-up phenomena from being generated on both of the channel sides simultaneously by the conventional method.
On the other hand, to prevent the charge-up phenomenon, it is also possible to reduce the amount of the ion beam without using the EFG, or to substantially reduce the amount per predetermined period by decreasing the duty ratio. However, when the dose of ion beams is relatively large, the throughput of the ion implantation is inevitably degraded. Accordingly, these methods are not so effective to solve these problems either.
FIG. 2 shows a cross section of a semiconductor substrate 64 for detecting electrical damage caused by conventional ion implantation, described above.
In fabrication of the semiconductor substrate 64, an SiO.sub.2 film 62 with a thickness of 200 .ANG. is first formed on a P type silicon (Si) substrate 61. A phosphorous-diffusion poly silicon film (Poly Si) 63 with a thickness of 4000 .ANG. is then formed on the SiO.sub.2 film 62. Thereafter, the Poly Si 63 is processed by the CDE method (chemical dry etching) , so as not to damage the substrate composition. The processed material is then annealed for about 60 minutes in a N.sub.2 gas atmosphere at 900.degree. C., so as to obtain the semiconductor substrate 64.
FIG. 3 is a diagram showing the relationship between a failure rate concerning insulating property and an applied electric field with respect to the substrate 64 subjected to conventional high-dose ion implantation. In the diagram, the horizonal axis shows the electric field applied to the poly Si 61 (where the Si substrate 61 is set at ground voltage), and the vertical axis shows the failure rate thereof.
As clearly seen from the same figure, when an EFG is not used for ion implantation, the insulating resistance of some resultant substrate 64 is destroyed under the condition of the electric field less than 1 MV/cm. The failure rate thereof is shown by the shaded portion 71, in FIG. 3.
On the other hand, when the current of the EFG is used in the predetermined range for ion implantation, the failure rate of the resultant insulating resistance is as high as that of the normal SiO.sub.2 film (8.5 to 9.5 MV/cm, shown by the shaded portion in FIG. 3).
But, when the current of the EFG is in the range greater than in the predetermined current range, the failure rate of the resulting insulating resistance of the substrate is in less than 1 MV/cm. Namely, the current of the EFG is so high that the amount of the electrons generated by it is large.
The proper range of the current applied to the EFG depends on the structure of the semiconductor substrate or the existence of the resist on the substrate can be determined.
When the current range applied to the EFG is narrow, the charge-up on the surface of the substrate is protected at one portion on the substrate, but it cannot be guarded at other portions. The amount of electrons generated by the EFG is not so large as to neutralize the positive electric charge on the surface of the substrate or is not lacking in order to delete it. In these cases, the substrate is charged with positive electric charge in the positive state, with electrons in the negative state.